The weekly register. [volume] (Point Pleasant, Va. [W. Va.]) 1862-1909, 1887-06-29
The Great Railroad bridge 10
Span the Big KnnaWhn.

done: The caisson consists of a work
ing chamber and roof, upon which
the masonry is built. The working
chamber is about twenty feet by
forty five feet in the clear, heavily
trussed and braced with iron rods
and 12x12 beams. The sides are OF
two thicknesses of 12x12 timbers,
with a double thickness of three inch
plank on the outside, all timbers
being drift bolted thoroughly to-
gether, and plank spiked to timbers.
The roof is of six courses of 12x12
timbers, laid alternately Longitude
nally and transversely. Through
the roof are left the requisite open
ings, first for the main shaft or air
lock, by which men are admitted
into the working chamber; secondly,
for the material lock, through which
the excavated material is brought
out of the working chamber, and
lastly for the pipes which carry in
the air, and those through which
loose sandy material is blown out.
In making these openings particular
care is taken to weaken the roof as
little as possible, for fear of slightre-
Election, causing seams to open and
therefore leakage. The air is driv-
en- into the working chamber
through pipe, by powerful COM
pressor, which with the donkey or
hoisting engine, boiler, pumps and
other necessary machinery, are in a
barge, moored alongside of caisson.
The air lock or main shaft is circu-
lar tube, about four feet in diameter,
strongly made in sections of boiler
plate, with diaphragms at proper in
tervals so that the height of lock may
be increased as the caisson sinks
deeper, or as the river rises. In
this air lock are two doors, some
eight feet apart, both of which have
stop cocks in them. Suppose the
air pressure to be on in the working
chamber and in the air lock to the
upper door when we wish to go in.
At a given signal the lock tender,
who stays inside, closes the lower
door and opens the stop cock in the
upper door. The compressed air
rushes out and equalizes itself with
that outside, when the upper door
falls; and we can now step in. The
upper door is then securely closed
and the compressed air turned in by
means of the stop cock in the lower
door. We are conscious of a very
disagreeable sensation in the ears
accompanied by a feeling as if we
wished we were somewhere else,
On. b. Any one wishing full par
ticulars as to their feeling will please
apply to (Mr. Perry Simpson, AS
soon as the compressed air has fully
filed the space between the doors
the lower door falls, and as the SEN
sation above referred to quickly
passes, we are at liberty to go below
and examine. Here we find the
men, who wear long-legged rubber
boots, but are otherwise as thinly
clad as possible, on account of the
heat, which is quite severe on hot
days. The material is loaded in a
large bucket and carried to the sur
face through the material lock-
Large logs, timber of all kinds, bowl
ders, gravel and mud, all have to be
gotten out. AS the excavation
goes on the weight of the masonry
which has been laid on the roof,
forces the caisson down, until it rests
on its final bed in the solid rock
After the sinking is complete the
working chamber is filed with con-
crete, which is rammed into every
part of it, thus making altogether a
structure as solid as the rock it
rests on. The depth to be reached
here is comparatively small, less
than twenty feet below low water,
but this method of foundation is sue
cessfully used at depths up to 100
feet. At this limit the men can no
longer stand the air pressure. The
pressure used here varies from ten
to fifteen pounds per square inch,
which is not enough to harm any one.
There is used in each caisson about
$50, 900 feet board measure timber,
40. 900 pounds iron, $50 cubic yards
concrete, and lastly about 100
bales of oakum, as every joint
has to be most thoroughly caulked
to prevent air and water leakage.